Continuum radiation as an electron density diagnostic in microwave-generated microplasmas

Summary form only given. The analysis of continuum emission radiation as a plasma diagnostic has primarily been employed for high-pressure arcs and inductively-coupled plasma torches in or near thermal equilibrium. Due to the high ionization fractions in such plasmas, the continuum spectrum is dominated by photons emitted from recombining electrons and ions¹. Under typical microplasma conditions of ionization fractions less than 10^-4 and electron temperatures on the order of 1 eV, however, the continuum spectrum consists primarily of Bremsstrahlung resulting from electron-neutral collisions. Spatial variations in continuum emission intensity in microplasmas can be directly correlated with line-of-sight electron densities. We verify this relationship by examining a microplasma using multiple electron density diagnostics. We present direct comparisons of relative continuum emission intensities with spatially-resolved electron densities from Stark broadening² as well as lumped electron densities from estimations of the plasma electrical impedance for a range of discharge conditions. All measurements are made for a single filamentary microplasma generated in high-pressure argon using a microstrip resonator design. Once an absolute calibration is made, the continuum radiation technique allows estimation of the electron density with lower noise in a wider range of plasma conditions than Stark broadening. The dependence of continuum radiation on the electron temperature is also considered.